Coating Defects

Craters

Areas in the coating surface where film thickness has been precipitously reduced, potentially to the substrate surface.

Craters

A blank area in the powder coat, which extends all the way to the substrate (diameter up to 2mm).

  • Potential Cause

    Insufficient pre-treatment (eg oil and grease residues).

  • Solution

    Test pre-treatment and ensure parts coming out of the system are clean.  Run a white cloth over the surface, it should not be stained.  If necessary contact pre-treatment supplier.

  • Potential Cause

    Rust on parts

  • Solution

    Assure clean surface, apply recommended pre-treatment

  • Potential Cause

    Incompatibility with powder coatings from other manufacturers, particularly for acrylic coatings.

  • Solution

    Cleaning of coating and application equipment, contact powder coating supplier

  • Potential Cause

    Surrounding air contaminated, especially from silicone oils.  Surprisingly, deodorants can cause this problem too.

  • Solution

    Balance air flows in plant, avoid cross drafts.  Discuss with operators whether they've changed deodorants.

  • Potential Cause

    Work piece moist/wet

  • Solution

    Increase drying time/temperature in dry-off oven

  • Potential Cause

    Liquid and powder paint in same plant

  • Solution

    Definitely avoid – reconfigure plant

  • Potential Cause

    Base part was cleaned with highly volatile solvents such as MEK or Cellulose (Standard) Thinners.

  • Solution

    Solvent rags quickly become saturated in oils that spread further surface oil rather than removing it.  Replace rags more frequently or consider another method of cleaning.

Pinholes

Very small craters, usually not extending down to the surface.

  • Potential Cause

    Coating is contaminated by an incompatible powder

  • Solution

    Clean application equipment when changing from one powder to the next.

  • Potential Cause

    Pinholes produced from out-gassing

  • Solution

    Adjust dry off oven temperature to ensure no moisture is on the part.  Check powder for oil or moisture introduced in reclaim, from the air supply or the virgin powder.  A thick film may cause out-gassing.  Reduce the film thickness to enable the gas to escape through the surface during cure.  Polyurethane powder coatings may emit a blocking agent.  Excessively high cure temperatures may make lower polymeric components within the coating volatile.  Note oven temperatures if pinholes form.

  • Potential Cause

    Zinc substrate, including hot dip galvanising or zinc metal spray, releasing gases trapped during the galvanising process.

  • Solution

    Zinc substrates are notorious.  Use a powder coating specifically formulated for zinc substrates; use a zinc sealer primer coat; preheat zinc above curing cycle temperature; add a ‘degassing modifier’

  • Potential Cause

    Unsatisfactory rinse stages

  • Solution

    Chemical from cleaning and pretreatment stages may be dried onto the surface, evaporating at higher cure temperatures.  Assess conditions of rinse stages.

  • Potential Cause

    Pinholes produced from contaminating emissions in the curing oven.

  • Solution

    Masking products are often plasticized to make the product soft and flexible.  Plasticizers can emit volatiles that may reduce the coating’s hardness and produce pinholes.  Monitor ovens and maintain appropriate air exchange.

< Back to Knowledge